1. Field of the Invention
The present invention relates in general to a dielectric ceramic composition, particularly to a dielectric ceramic composition suitable for forming dielectric resonators, for example, which are used with microwave (high-frequency) devices.
2. Discussion of the Prior Art
Keeping pace with recent developments of various microwave devices such as pocket or portable telephones, there has been a growing demand for small-sized high-performance dielectric resonators used in such devices. In particular, a dielectric ceramic used to form such dielectric resonators is required to have the following physical and structural features;
(1) The dielectric ceramic should have a sufficiently high specific dielectric constant (.tau.r), to reduce the size of a resonator when the resonator is operated at a given frequency.
(2) The dielectric ceramic should have a sufficiently high Q value, to reduce the transmission loss of the resonator formed of the dielectric ceramic. PA1 (3) The temperature coefficient (.tau.f) of the resonance frequency of the resonator formed of the dielectric ceramic should be sufficiently low, to minimize changes of the operating characteristics of the resonator with respect to temperature.
There have been known various dielectric ceramic compositions used for forming such dielectric resonators. For example, laid-open Publication No. 58-20905 of examined Japanese Patent Application discloses a dielectric ceramic composition whose major components consist of BaO and TiO.sub.2, a portion of which may be substituted by another element or other elements. A dielectric ceramic formed of the disclosed composition has a relatively low dielectric constant of around 30.about.40, and a relatively low Q value. Another example of dielectric ceramic composition is disclosed in laid-open Publication No. 59-23048 of examined Japanese Patent Application, which has a composite perovskite structure such as Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3. The dielectric ceramic formed of this composition has a relatively high Q value, but suffers from a relatively low dielectric constant of around 30.about.40.
There is also known a dielectric ceramic composition as disclosed in laid-open Publication 56-102003 of unexamined Japanese Patent Application, whose major components consist of BaO, TiO.sub.2, Nd.sub.2 O.sub.3 and Bi.sub.2 O.sub.3. While this dielectric ceramic composition has a relatively high dielectric constant, the temperature coefficient of the resonance frequency of the resonator formed of this ceramic composition is undesirably high, making it difficult for the resonator to provide satisfactory operating characteristics. A further example of dielectric ceramic composition as disclosed in laid-open Publication No. 57-21010 of unexamined Japanese Patent Application, whose major components consist of BaO, TiO.sub.2, Nd.sub.2 O.sub.3 and Sm.sub. O.sub.3, cannot achieve a sufficiently low temperature coefficient of the resonance frequency and a sufficiently high dielectric constant at the same time. Still another example of dielectric ceramic composition is disclosed in laid-open Publication No. 62-100906 of unexamined Japanese Patent Application, which contains BaO, TiO.sub.2, Nd.sub.2 O.sub.3, Sm.sub.2 O.sub.3 and Bi.sub.2 O.sub.3 as major components. The dielectric constant of this ceramic composition is not so high as to allow the resonator using the composition to be sufficiently small-sized. Further, this composition is economically disadvantageous since it contains as much as 7-20 mole % of samarium oxide which is a relatively expensive material.
To improve the above-described properties of the dielectric ceramics, there have been proposed to add alumina (Al.sub.2 O.sub.3) to the known dielectric ceramic composition. For example, Publication No. 61-41863 of examined Japanese Patent Application discloses a dielectric ceramic composition which contains BaO, TiO.sub.2, Sm.sub.2 O.sub.3 and Al.sub.2 O.sub.3 as major components. The added alumina is effective to raise the dielectric constant and Q value of the dielectric ceramics obtained. However, the addition of alumina to this composition also results in an increase in the temperature coefficient. Another example of dielectric ceramic composition is disclosed in laid-open Publication No. 1-227303 of unexamined Japanese Patent Application, which contains BaO, TiO.sub.2, Nd.sub.2 O.sub.3 and Al.sub.2 O.sub.3 as major components. The dielectric ceramic formed of this composition has a relatively low Q value due to the addition of alumina, resulting in an increased transmission loss of the resonator obtained from the dielectric ceramic.
A further example of dielectric ceramic composition as disclosed in laid-open Publication No. 63-117958 of unexamined Japanese Patent Application, whose major components consist of BaO, TiO.sub.2, Nd.sub.2 O.sub.3, Bi.sub.2 O.sub.3 and Al.sub.2,O.sub.3, has a relatively high dielectric constant. The temperature coefficient of this ceramic composition is stabilized by the addition of alumina, but still remains relatively high. Still another example of dielectric ceramic composition as disclosed in laid-open Publication No. 1-227304 of unexamined Japanese Patent Application, whose major components consist of BaO, TiO.sub.2, Nd.sub.2 O.sub.3, Bi.sub.4 Ti.sub.3 O.sub.12 and Al.sub.2 O.sub.3, also has a relatively high dielectric constant. However, the dielectric ceramic formed of this composition has a relatively low Q value due to the addition of alumina, resulting in an increased transmission loss of the resonator obtained from the dielectric ceramic.